1. Field of the Invention
The present invention relates to apparatus for facilitating electroplating and, more particularly, to a wafer holder for use in electroplating wafers and other such substrates.
2. Description of Related Art
The fabrication of microcircuits requires the precise positioning of a number of appropriately doped regions in a slice of semiconductor, which positioning is followed by effectuation of one or more interconnection patterns. These appropriately doped regions typically include a variety of diffusions and implants, cuts for metallizations and gates, and windows in protective cover layers through which connections can be made to bonding pads. For each of these regions a sequence of steps is required, together with a specific pattern layout.
A common method of patterning heretofore has involved a photolithographic transfer followed by etching. As is well known to those skilled in the art, photolithography effects transfer of a desired pattern onto the surface of a silicon wafer by selectively allowing light to strike a thin film of photosensitive material coated on the wafer, certain of which material can then be locally removed based upon its solubility, changed or unchanged, after exposure to the light. Removal of material from areas unprotected by the photosensitive material or "photoresist" is accomplished in an etching step. The etching processes used in integrated circuit ("IC") fabrication can take place either in a liquid ("wet etching") or gas ("dry etching") phase. These processes can also be purely physical (e.g., wherein material is removed by bombardment which high-energy ions), purely chemical (e.g., wherein material is removed by dissolution), or a combination of both (e.g., wherein material is removed by bombardment with reactive ions which also react chemically with the etched material). Recognizing that all etching processes may be characterized by their selectively (i.e., in materials attacked by the etching agent) and degree of anisotropy (i.e., etching in one direction only, as opposed to isotropic etching, wherein material is removed at the same rate in all directions), it should be appreciated that all etching processes involve some degree of compromise in selectivity, anisotropy, or both selectivity and anisotropy.
As it has become desired to create increasingly accurate and dense pattern geometries, those skilled in the art have searched for methods of patterning that lack the "bias-type" compromises of etching processes. One such method that has been and is still being developed is electroplating, that is, the electrodeposition of an adherent coating upon an object. Although electroplating has long been used in patterning printed circuit boards, its use in patterning high density features onto wafers and substrates is still relatively new. One of the advantages of additive patterning approaches, such as pattern electroforming, over subtractive methods, such as etching, that has been discovered is that very little bias in dimension occurs with electroforming and therefore very accurate and dense geometries can be fabricated.
Although electroplating may become a favored technique for patterning high density features onto wafers and substrates, it has heretofore had a number of shortcomings and deficiencies. One of these deficiencies is that thickness variation across a work piece or from item to item is difficult to control. In the printed circuit board industry or in surface finishing industries, the control of plating thickness is not as critical as it is in the industries fabricating high interconnect density substrates or fabricating input/output bond pads. In the latter two types of industries, needless to say, the requirements for controlled and uniform plate thickness are very important.
A problem in plating thickness control is that the local plating rate is dependent not only on the plating bath chemistry and the plating process parameters but also on the geometry and pattern to be plated. For example, there is a general tendency for higher plating rates at corners and edges because higher electric field densities exist in these areas. In pattern plating complex geometries with varying pattern demographics, the electric flux distribution across a wafer or substrate can be very non-uniform.
Another shortcoming and deficiency of electroforming as an approach for patterning wafers and high density interconnect substrates is that very little commercially available equipment exists, so that companies that wish to investigate electroplating of delicate parts such as wafers and interconnect substrates need to develop their own equipment.